WASHINGTON -- After directly sampling the high-altitude exhaust from a Concorde supersonic jet for the first time, a research team has warned that a new fleet of such planes could pose more danger to the Earth's protective ozone layer than previously believed.

The measurements were made last October by a civilian version of the U-2 spy plane that trailed a chartered Air France Concorde flying off the coast of New Zealand at 53,000 feet.

The exhaust of the supersonic transport contained more microscopic particles -- a mix of soot, water and sulfuric acid -- than had been expected, according to a report published in Friday's issue of the journal Science.

If a fleet of 500 next-generation SSTs were built -- an economic and technical issue being studied by governments and aircraft manufacturers -- the planes could persistently produce enough particles to equal the levels seen after small volcanic eruptions, the researchers say.

Those exhaust particles could promote chemical reactions that might destroy significantly more ozone than has been predicted if only the ozone-depleting nitrogen oxides in aircraft exhaust are considered.

The researchers did not try to estimate the total potential ozone loss. But David Fahey, a team leader, said in a telephone interview that a fleet of 500 SSTs with exhausts comparable to the Concorde might reduce global ozone levels by perhaps 2 percent.

The ozone layer helps shield the Earth from ultraviolet radiation. By one estimate, each 1 percent drop in ozone could increase the incidence of nonmelanoma skin cancer worldwide by 2 percent.

Mr. Fahey, who is with the National Oceanic and Atmospheric Administration's Aeronomy Laboratory in Boulder, Colo., said the new study should spur more analyses of upper atmosphere chemistry and the possible steps to counter ozone loss, including use of low-sulfur fuel.

"I think it's a big caution flag," Mr. Fahey said. But he said the discovery of abundant particles in the Concorde exhaust should not be a showstopper for advanced SST development.

"If we are correct, and these particles are produced by highly oxidized sulfur in the fuel," Mr. Fahey said, "removing sulfur in the fuel will improve the situation."

The team's ER-2 chase plane, operated by the National Aeronautics and Space Administration's Ames Research Center in California, carried instruments capable of scooping up air samples and "sniffing" them for traces of exhaust gases and particles.